I have a 15 gallong expansion tank that I need to drain. The problem I having is, when I close the stop/wastevalve between the tank and boiler and open the tank drain valve no water comes out. It seem that there is notenough air in the tank to let it drain. I looked but didn't see any kind of air value on the tank to let in some air.

Any suggestions what I can do? When I open the stop/waste valve and the tank drain valve water will drain outbut now I'm also draining water from the entire system.